1. Field of the Invention
The present invention relates to a radiation imaging apparatus, and particularly relates to a radiation imaging apparatus that can incorporate an anti-scatter grid and a photo timer.
2. Description of the Related Art
Recently, digital radiation imaging apparatuses have been put into practice that directly digitalize a radiological image using radiation detector panels in which fluorescent substances are in close contact with large area solid-state image sensors, that is, so-called flat panel detectors (FPDs). Such digital radiation imaging apparatuses can immediately obtain a radiological image as digital information, and, thus, many advantages are provided such as reducing the amount of effort required for an imaging operation by an engineer or improving the efficiency in radiogram interpretation by a doctor.
Furthermore, in commonly performed radiography of the chest, the abdomen, or the like, imaging using an anti-scatter grid (hereinafter, referred to as a “grid”) is performed in order to improve the contrast in a radiological image. There are a plurality of types of grids having different grid properties such as grid density, grid ratio, or focusing distance. In clinical practice, these grids are used in different manners in accordance with an object to be imaged. When a proper grid is selected and used, it is possible to obtain a high-contrast sharp image useful for diagnosis, and to prevent a patient from being overexposed to radiation and the irradiation time from being long. Meanwhile, in the case of objects to be imaged where the amount of scattered radiation generated is small, such as bones of extremities or infants, imaging is performed without using a grid.
An imaging apparatus has been developed in which a grid can be easily replaced by a plurality of grids having different properties and an imaging mode can be changed so as not to use a grid in response to a request to change imaging methods in this manner. Japanese Patent Laid-Open No. 2001-154299 (hereinafter, referred to as a “Document 1”) discloses a configuration in which a grid is contained inside an imaging apparatus and can be attached and detached from the outside of the imaging apparatus. Furthermore, in the imaging apparatus of Document 1, the contained grid is conveyed by a drive mechanism configured from a motor and a cam. This configuration prevents grid stripes from being reflected on a radiological image. FIG. 7 is a vertical cross-sectional view of a schematic configuration of an ordinary radiation imaging apparatus having means for incorporating a grid into the imaging apparatus.
A radiation imaging apparatus 1 has a housing formed so as to contain internal components by combining two separate pieces; namely a cover member 2 functioning as a front cover that is brought into contact with a patient and a frame member 3 functioning as a rear cover. It is possible to access components inside the housing by detaching the cover member 2 from the frame member 3.
The imaging area on which radiation is incident on the cover member 2 is formed by a carbon plate 4 having a high X-ray transmission. Imaging is performed by pressing an area to be imaged (e.g., the chest) of a patient against the carbon plate 4. Accordingly, the cover member 2 and the carbon plate 4 are required to be strong enough to withstand the load applied by the patient. Ordinarily, the rigidity is secured by using a strong material for the cover member 2 or increasing the thickness. Furthermore, the strength of the carbon plate 4 is increased by fixing the carbon plate 4 using a reinforcing plate 5 or the like.
The radiation that has been transmitted through the carbon plate 4 passes through a grid member 6 that is disposed behind the carbon plate 4, and, thus, undesired scattered radiation diffusely reflected from the patient is removed. The grid member 6 is configured from a grid alone or a combination of a grid and a frame that surrounds and holds the grid. The grid member 6 is held by a grid containing member 7, and the grid containing member 7 is fixed to the frame member 3. The grid containing member 7 allows the grid member 6 to slide forward and backward (in directions perpendicular to the section of the diagram) for removal from the radiation imaging apparatus 1. A side face of the cover member 2 is provided with an opening through which the grid member 6 is to be inserted and removed, and the grid is attached to and detached from the radiation imaging apparatus 1 or replaced by a grid having another specification. Furthermore, in an imaging apparatus having a movable grid mechanism, a drive unit 10 for conveying the grid is attached to the frame member 3. The grid member 6 is conveyed along the guide of the grid containing member 7 by the actuating force from the drive unit 10.
Meanwhile, recently, means for performing imaging while keeping a grid still, obtaining radiological information containing grid stripes, and removing the grid stripes by the subsequent image processing has been also put into practice as new image processing algorithms are developed and processing speed is increased. An imaging apparatus using this sort of stationary grid is provided with positioning means for fixing the grid member 6 to a predetermined position on the grid containing member 7. The radiation that has been transmitted through the grid member 6 is incident on a radiation detection unit 8 constituting a radiation detector panel, and converted into digital information. The obtained signals are transmitted to image processing means, where an image used for diagnosis is generated.
Furthermore, in an ordinary radiation imaging apparatus, a photo timer has been conventionally used in order to obtain an image with an appropriate density by preventing underexposure or overexposure. A photo timer is an apparatus for blackening an obtained image to a constant degree and measures the dose of X-rays transmitted through an object in order to adjust the imaging time. A photo timer is also referred to as an auto exposure control (AEC). It is possible to simplify preset of imaging conditions by attaching the photo timer. A photo timer light receiving unit is a thin rectangular plate (film) having a thickness of 3 mm or less, and its receptor field is provided with a photo multiplier. The light receiving unit is disposed ordinarily behind the grid and in front of the sensor when viewed from the X-ray tube. If the dose of X-rays detected by the photo timer or the light receiving unit exceeds a set value, a signal is generated in order to stop radiation irradiation. Here, the shape or the position of the light receiving face (receptor field) varies depending on manufacturers. In recent digital imaging, the density can be corrected after imaging, and, thus, the demand for a photo timer for adjusting the exposure is lowered. However, since there is a strong demand for reducing exposure of a patient to radiation, use of a photo timer for preventing overexposure to radiation is becoming more important.
In order to properly control radiation irradiation using a photo timer, the correlation between the radiation dose obtained by the photo timer light receiving unit and the radiation dose obtained by the radiation detection unit 8 has to be high. Accordingly, the photo timer light receiving unit has to be disposed behind a grid where attenuation of incident radiation is high. In FIG. 7, the photo timer is disposed at a space 9 between the grid member 6 and the radiation detection unit 8.
Here, as a photo timer incorporated in a radiation imaging apparatus, photo timers having different specifications or shapes are requisite according to different manufacturers of X-ray generating apparatuses controlling radiation irradiation or according to different apparatus models even in the case of the same manufacturer. Accordingly, the operation that attaches a photo timer and adjusts connection with an X-ray generating apparatus cannot be performed at the time of shipment of a radiation imaging apparatus, and the operation that attaches the photo timer to the inside of the apparatus is performed by an engineer on site. As described above, the photo timer is attached to the inside of the radiation imaging apparatus so as to be positioned behind the grid, and, thus, in order to attach the photo timer, grid mechanism components have to be detached first. This process requires extra effort for an installation operation and a maintenance operation after the installation. Furthermore, there is another problem in that the durability and the strength of the apparatus tend to be lowered as the components are more frequently attached and detached as a result of the attachment of the photo timer.
Furthermore, medical apparatuses have to be prevented from going out of order through proper maintenance in order not to cause trouble to hospitals and patients, and have to be immediately restored when they are out of order. In the radiation imaging apparatus 1, the accessibility to the radiation detection unit 8, which is a main unit for obtaining an image, and the replaceability of the components are important. Conventionally, the radiation detection unit 8 is positioned behind the grid mechanism components and the photo timer components, and a large number of operation steps are necessary for accessing the radiation detection unit, which is an obstacle to a reduction in downtime.